1. Field of the Invention
The present invention relates to a rotating electrical machine such as a motor or a generator. More particularly, the present invention relates to an axial gap rotating electrical machine in which a disc-shaped rotor and a stator are oriented opposing one another in the axial direction.
2. Description of the Related Art
A so-called axial gap motor is known in which a stator is arranged opposing end surfaces, in the axial direction, of a disc-shaped rotor across an air gap. This motor obtains rotational driving force from magnetic force acting between the surfaces of the axially opposing rotor and stator. The axial gap motor is advantageous in that it can be made thinner in the axial direction than a so-called radial type motor which has a conventional cylindrical rotor and an annular stator that surrounds the peripheral surface of the rotor.
Presently several types of rotors for axial gap motors are known: a reluctance type having asperities of magnetic material on an end surface that faces a stator, a permanent magnet type which has N-poles and S-poles corresponding to rotational-driving magnetic poles of the stator, and an inducting body type in which inductor rods are aligned in the radial direction (see JP(A) 10-80113, paragraph 0022). In order to construct a thin motor with one rotor and a pair of stators sandwiching the rotor, the motor disclosed in JP(A) 10-80113 employs a structure in which salient poles are arranged on one side of the rotor, and permanent magnets are arranged on the other side of the rotor so that their magnetization surfaces (magnetic poles) face in the axial direction of the rotor. As a result, reluctance torque is generated on the side having the salient poles and permanent magnetic torque is generated on the side having the permanent magnets.
The applicant invented a structure in which cores and permanent magnets are alternately arranged in the circumferential direction on one side of the rotor, with the permanent magnets oriented so that their magnetization surfaces face in the circumferential direction of the rotor. As a result, both reluctance torque and permanent magnet torque are able to be generated on one side of the rotor. Thus this structure makes it possible to increase the torque able to be generated on one rotor side compared with the structure disclosed in JP(A) 10-80113 (hereinafter simply referred to as the “related art”), such that higher torque is able to be generated than is generated with the axial gap motor of the related art.
With this kind of structure in which the magnetization surfaces of the permanent magnets face in the circumferential direction of the rotor, however, the magnetic path from the permanent magnets passes through the cores adjacent to the permanent magnets. As a result, the width of the magnetic path becomes narrower on the inner circumferential side of the rotor than it is on the outer circumferential side, so that the magnetic path at the outer periphery and magnetic path at the inner periphery are uneven. When the magnetic path is narrow at the inner periphery and wide at the outer periphery in this way, the magnetic flux vector from the permanent magnets is not in the circumferential direction, but rather points toward the outer periphery. When a component in the radial direction of the rotor is generated in the magnetic flux vector, that component does not contribute to the driving force which rotates the rotor, so the magnetic force of the permanent magnets cannot be sufficiently reflected in the torque.
Furthermore, with a structure in which the magnetization surfaces of the permanent magnets face in the circumferential direction of the rotor, the magnetic path on the inner circumferential side at the core portion is narrow, resulting in the magnetic flux density becoming too large on the inner circumferential side. As a result, magnetic saturation occurs due to the characteristics of the electromagnetic steel plates used as the core material. When magnetic saturation occurs in this way, the amount of magnetic flux with respect to the amount (volume) of the magnet decreases. As a result, the permanent magnets are unable to be used effectively, which leads to problems such as increases in weight and cost.
In the foregoing related art, the permanent magnets are arranged so that their magnetization surfaces face in the axial direction of the rotor, and there is a back yoke which forms the magnetic path on one of the magnetization surfaces so the aforementioned problems do not occur. However, because the magnetic poles of the permanent magnets face in the direction of the rotational axis, torque as high as that obtained by the foregoing structure according to the invention of the applicant is unable to be output.